In recent years there has been a growing interest in increased automation in the control of the drive train of motor vehicles, and most especially in control of the drive train of large trucks. The use of automatic transmissions in passenger automobiles and light trucks is well known. The typical automatic transmission in such a vehicle employs a fluid torque converter and hydraulically actuated gears for selecting the final drive ratio between the engine shaft and the drive wheels. This gear selection is based upon engine speed, vehicle speed and the like. It is well known that such automatic transmissions reduce the effectiveness of the transmission of power from the engine to the drive shaft, with the consummate reduction in fuel economy and power as compared with the skilled operation of a manual transmission. Such hydraulic automatic transmissions have not achieved wide spread use in large motor trucks because of the reduction in efficiency of the operation of the vehicle.
One of the reasons for the loss of efficiency when employing a hydraulic automatic transmission is loss occurring in the fluid torque converter. A typical fluid torque converter exhibits slippage and consequent loss of torque and power in all modes. It is known in the art to provide lockup torque converters that provide a direct link between the input shaft and the output shaft of the transmission above certain engine speeds. This technique provides adequate torque transfer efficiency when engaged, however, this technique provides no gain in efficiency at lower speeds.
It has been proposed to eliminate the inefficiencies inherent in a hydraulic torque converter by substitution of an automatically actuated friction clutch. This substitution introduces another problem not exhibited in the use of the hydraulic torque converters. The mechanical drive train of a motor vehicle typically exhibits considerable torsional compliance in the driveline between the transmission and the traction wheels of the vehicle. This torsional compliance may be found in the drive shaft between the transmission and the differential or the axle shaft between the differential and the driven wheels. It is often the case that independent design criteria encourages or requires this driveline to exhibit considerable torsional compliance. The existence of substantial torsional compliance in the driveline of the motor vehicle causes oscillatory response to clutch engagement. These oscillatory responses can cause considerable additional wear to the drive train components and other parts of the vehicle. In addition, these oscillatory responses can cause objectionable passenger compartment vibrations.
The oscillatory response of the driveline to clutch engagement is dependent in large degree to the manner in which the input speed of the transmission, i.e. the speed of the clutch, approaches the engine speed. A smooth approach of these speeds, such as via a decaying exponential function, imparts no torque transients on clutch lockup. If these speeds approach abruptly, then a torque transient is transmitted to the driveline resulting in an oscillatory response in the vehicle driveline.
Thus it would be an advantage to provide automatic clutch actuation of a friction clutch that reduces the oscillatory response to clutch engagement. The problem of providing such automatic clutch actuation is considerably increased in large trucks. In particular, large trucks exhibit a wide range of variability in response between trucks and within the same truck. The total weight of a particular large truck may vary over an 8 to 1 range from unloaded to fully loaded. The driveline compliance may vary over a range of about 2 to 1 among different trucks. Further, the clutch friction characteristic may vary within a single clutch as a function of degree of clutch engagement and between clutches. It would be particularly advantageous to provide such an automatic clutch actuation system that does not require extensive adjustment to a particular motor vehicle or the operating condition of the motor vehicle.